1. Field of the Invention
The present invention relates to an oil pump of the type which includes an outer rotor contained within a pump housing and an inner rotor coupled within the outer rotor and mounted on an input shaft for rotation therewith.
2. Description of the Prior Art
Disclosed in Japanese Patent Laid-open Publication No. 63(1988)-223382 is an oil pump of the type which includes a pump housing, an inner rotor formed at its inner periphery with a pair of diametrically opposed radial projections for engagement with an input shaft and at its outer periphery with a plurality of external teeth and being mounted on the input shaft within the pump housing for rotation therewith in a condition where opposite side faces of the inner rotor are slidable in contact with a pair of axially opposed internal side faces of the pump housing, and an outer rotor formed at its inner periphery with a plurality of internal teeth for engagement with the external teeth of the inner rotor and being coupled within a cylindrical recess formed in the pump housing for rotation about a rotation center eccentrically displaced from the rotation center of the inner rotor. In the oil pump, the inner rotor is formed at one side of the radial projections thereof with a cylindrical protrusion which is coupled within an axial bore in the pump housing.
Although the cylindrical projection of the inner rotor is useful to position the inner rotor in place, sliding engagement of the cylindrical projection with an inner periphery of the bore in the pump housing results in an increase of torque loss of the oil pump. In addition, it is required to form an annular sealing surface on one side of the inner rotor at the outer periphery of the cylindrical projection. As a result, the inner rotor is enlarged in diameter, causing an increase of torque loss of the oil pump.